/**

   *

   * @param job  配置信息

   * @param split  split

   * @param recordDelimiter   分列的字符

   * @throws IOException

   */

  public LineRecordReader(Configuration job, FileSplit split,

      byte[] recordDelimiter) throws IOException {

    //如果没有配置每一行的最大长度，那么每行默认的最大长度为Integer类型的最大值

    this.maxLineLength = job.getInt(org.apache.hadoop.mapreduce.lib.input.

      LineRecordReader.MAX_LINE_LENGTH, Integer.MAX_VALUE);

    start = split.getStart();

    end = start + split.getLength();

    final Path file = split.getPath();

    //默认io.compression.codecs  gzip

    compressionCodecs = new CompressionCodecFactory(job);

    codec = compressionCodecs.getCodec(file);

    // open the file and seek to the start of the split

    final FileSystem fs = file.getFileSystem(job);

    fileIn = fs.open(file);

    if (isCompressedInput()) {

      decompressor = CodecPool.getDecompressor(codec);

      if (codec instanceof SplittableCompressionCodec) {

        final SplitCompressionInputStream cIn =

          ((SplittableCompressionCodec)codec).createInputStream(

            fileIn, decompressor, start, end,

            SplittableCompressionCodec.READ_MODE.BYBLOCK);

        in = new CompressedSplitLineReader(cIn, job, recordDelimiter);

        start = cIn.getAdjustedStart();

        end = cIn.getAdjustedEnd();

        filePosition = cIn; // take pos from compressed stream

      } else {

        in = new SplitLineReader(codec.createInputStream(fileIn,

            decompressor), job, recordDelimiter);

        filePosition = fileIn;

      }

    } else {

      //从打开的文件找到指定位置

      fileIn.seek(start);

      in = new SplitLineReader(fileIn, job, recordDelimiter);

      filePosition = fileIn;

    }

    // If this is not the first split, we always throw away first record

    // because we always (except the last split) read one extra line in

    // next() method.

    //如果不是第一个dplit，那么将start设置成当前位置+第一行的长度。

    //跨过第一行，因为第一行数据已经在上一个split中读取了

    if (start != 0) {

      start += in.readLine(new Text(), 0, maxBytesToConsume(start));

    }

    this.pos = start;

  }